1. Field of the Invention
The present invention relates to a control device, for a die cushion mechanism, which generates a force on a slide of a press machine using a servomotor as a drive source and, in particular, to a control device for controlling the force generated by the die cushion mechanism of the press machine.
2. Description of the Related Art
It is known that a press machine, for press working such as bending, drawing or punching, can be provided with a die cushion mechanism, as an attached device, for applying a predetermined force or pressure, during the press working, to a movable support member (generally called a slide) supporting a first mold for press working, the force being generated by another movable member supporting a second mold. The die cushion mechanism is generally configured such that the slide (or the first mold), moving in a mold-clamping direction, directly or indirectly collides with a movable element (generally called as a cushion pad) held at a predetermined pressure and, until the molding is finished, the cushion pad is moved with the slide while applying force or pressure to the slide. During this operation, it is possible to prevent the occurrence of wrinkles in a workpiece to be pressed by, for example, clamping an area around a site, of the workpiece, to be pressed between the cushion pad and the slide.
Many conventional die cushion mechanisms use a hydraulic or a pneumatic unit as a drive source. However, control by a hydraulic or a pneumatic unit may be carried out only under a constant pressure. It is preferable that the pressure during drawing is varied corresponding to the amount of the drawing. However, the pressure cannot be varied in a hydraulic or a pneumatic unit.
In recent years, therefore, a die cushion mechanism using a servomotor as a drive source has been used to carry out force control at high speed, as described in Japanese Unexamined Patent Publication (Kokai) No. 10-202327. In the die cushion mechanism described in this publication, a cushion pad positioned below a slide of a press machine may be upwardly and downwardly moved by a servomotor, corresponding to the rise and fall motions of the slide. The servomotor is activated by a predetermined force command corresponding to the position of the cushion pad and adjusts the force or pressure applied, to the slide, by the cushion pad while moving the cushion pad with the slide. The collision of, and pressure between, the slide and the cushion pad may be determined by detecting a load applied to an output axis of the servomotor via the cushion pad.
In the press machine using the servo die cushion having a servomotor as a drive source, a method capable of performing an interconnected control of the slide and the die cushion has not yet been developed. Accordingly, a control device for the slide and a control device for the die cushion are separately arranged and operated. For example, the slide and the die cushion are individually controlled such that each of them is positioned at a predetermined position at each instant of time. As a result, a desired press force between the slide and the die cushion may be generated. In such a control, even when an alarm informing a malfunction of one of the slide and the die cushion is detected, the other of them is controlled based on a predetermined normal position command without reference to the alarm. Therefore, the moving die cushion is immediately stopped by the alarm and, for example, the motion of the slide toward the die cushion may be continued. As this causes an excess increase of the press force, the machine may be damaged by such a control.